1. Field of the Invention
The present invention relates to a hydraulic control apparatus for controlling a power transmitting system for a motor vehicle, including a hydraulically operated continuously variable transmission of belt-and pulley type, and a torque converter equipped with a lock-up clutch.
2. Discussion of the Related Art
There is known a hydraulic control apparatus for controlling a power transmitting system incorporating a torque converter equipped with a lock-up clutch, and a belt-and-pulley type continuously variable transmission whose speed ratio is continuously variable. The continuously variable transmission includes a first and a second shaft, a pair of variable-diameter pulleys provided on the first and second shafts, respectively, a belt connecting the pulleys for transmitting power therebetween, and a pair of hydraulic actuators for changing effective diameters of the pulleys. The hydraulic control apparatus is adapted to generate a belt tensioning pressure for controlling the tension of the belt. Further, the hydraulic control apparatus includes a lock-up clutch control valve for controlling pressures to be applied to engaging and releasing chambers of the lock-up clutch, to selectively place the clutch in its engaged or released position.
An example of such a hydraulic control apparatus is disclosed in laid-open Publication No. 57-161360 of unexamined Japanese Patent Application. In the control apparatus disclosed in this publication, a pressure of a working fluid used for engaging or releasing the lock-up clutch is applied to a pressure regulating valve for the belt tensioning pressure indicated above, such that the belt tensioning pressure is raised when the lock-up clutch is released, while the same pressure is reduced when the clutch is engaged. In this arrangement, a torque capacity of the continuously variable transmission is advantageously increased when the torque converter performs a torque amplifying function (i.e., when the lock-up clutch is released), so that the transmission receives a sufficiently large input torque from the torque converter. When the torque converter is not in the position for performing the torque amplifying function (i.e., when the lock-up clutch is engaged), the tension of the belt is controlled to a required minimum value, so that the belt engages the pulleys with a required minimum force.
In the hydraulic control apparatus of the above type, the lock-up clutch is deliberately slipped for a given period of time, to mitigate shocks upon engagement of the clutch, so that the clutch may be smoothly engaged. When this slip control operation is effected, the torque converter may continue to effect the torque amplifying operation during a period from a point at which the lock-up clutch control valve is operated to the engaging position for engaging the clutch, to a point at which the clutch is fully engaged. Therefore, if the belt tensioning pressure is reduced when the lock-up clutch control valve is placed in the engaging position, the belt may undesirably slip on the pulleys due to the torque amplifying operation of the torque converter, until the engaging action of the clutch is completed.